1. Field of the Invention
The present invention concerns electronic amplifiers working at high frequency. The term "high frequency" is used herein to mean frequencies which are, in principle, higher than one megahertz but, more generally, frequencies which are sufficiently high, in view of the stray reactances of the active components used in the amplifier, to entail the risk of self-oscillation in the amplifer.
A risk of self-oscillation appears as soon as the stray reactances cause a re-injection, towards the input of the amplifer, of an excessively high fraction of the output signal, and as soon as this fraction is added to the original input signal.
The invention is particularly applicable to amplifiers for which the main active component is a vacuum electronic tube (a grid tube such as a triode, a tetrode, a pentode, etc) and shall be described in detail with reference to this type of amplifier. But it can be applied more generally to other types of amplifiers, including transistor amplifiers.
2. Description of the Prior Art
The risks of self-oscillation in a triode amplifier, in an assembly where the cathode of the triode is grounded, are dee essentially to the existence of a stray capacitance between the control gate and the anode of the triode. Similarly, risks of undesirable self-oscillation appear in a transistor amplifier owing to stray junction capacitances between the base of the transistor, on the one hand, and the collector and emitter on the other hand. The same is true for field-effect transistors because of capacitances between the control gate and the source and drain.
The following description shall refer solely to assemblies with grid tubes without said description being in any way restrictive, and the invention is applicable provided that the amplifier comprises an active component having at least three electrodes, one of which receives a high frequency input signal to be amplified, another gives an amplified high frequency signal, and a third is used notably to define a reference potential for the other two electrodes, at least as regards high frequency.
FIG. 1 shows, by way of an example, a triode amplifer in which no precaution has been taken to neutralize risks of self-oscillation.
The amplifier has a triode 10 with a cathode K connected to a ground M, not only for AC currents but also for DC current. A gate G receives the high frequency signal to be amplified, and an anode A gives an amplified high frequency signal. The gate G is connected to an input E coupled to a previous stage by connecting and tuning circuits (not shown) depending on the application envisaged. Similarly, the anode A is connected to an output S connected to a following stage by connecting or tuning circuits (not shown) depending on the application considered. By construction, given the proximity of the different electrodes to one another, there is a stray capacitance Cgk between the gate and the cathode and a stray capacitance Cga between the gate and the anode.
The signal input is done by means of an input circuit which can be tuned to a range of frequencies to be amplified. In the example shown, the tuned input circuit comprises an inductive element L1 in parallel with a capacitive element C1. The value of the inductive element and that of the capacitive element can be adjusted to set the amplifer as a function of the frequency to be amplified. This circuit L1, C1, is connected between the gate and the ground by means of an uncoupling capacitive element Cd1, the value of which is high enough for its impedance to be negligible as compared with that of the other elements of this circuit at the frequencies considered. Thus, the circuit L1, C1 may be considered to have a terminal which is virtually grounded for the high frequency currents. A negative bias voltage -Vg is brought to the gate G through the inductive element L1.
Another tuning circuit, comprising an inductive element L2 in parallel with the capacitive element C2, is connected, firstly, to the anode A and, secondly, through an uncoupling capacitive element Cd2, having the same role as the capacitive element Cd1, to the ground M. The inductive element L2 or the capacitive element C2 can be adjusted to achieve the frequency tuning at the output of the amplifier. A high supply voltage Vht is applied to the anode through the inductive element L2.
The circuit thus described constitutes only one embodiment. Other frequently used assemblies comprise a signal input at the cathode and not at the gate, the gate being grounded for the high frequency current by an uncoupling capacitive element similar to the capacitive element Cd1. Certain assemblies even use a signal input at the gate and an output at the cathode, the anode being grounded. Similar circuits could be described wherein the active component is a tetrode o a pentode. All these circuits are subject to risks of self-oscillation due to stray capacitances between the output and the input. The method most usually employed to neutralize this risk (called the neutrodyning operation) consists in placing inductive circuit elements in parallel on the troublesome stray capacitance, (the capacitance Cga in FIG. 1 because it brings a fraction of the output signal present at the anode directly to the gate). These inductive circuit elements are elements such that the association of the capacitive element and inductive element in parallel forms a anti-resonant circuit at the working frequency, namely a circuit with very high impedance (infinite impedance in theory), where the ohmic losses are overlooked. The signal fraction re-injected from the output (anode) towards the input (gate) will be all the more negligible as this impedance between the anode and the gate will be high.
This method leads to a diagram such as the one shown in FIG. 2. In this figure, it is seen that a variable inductive element L3 has been added, series-mounted with a capacitive element Tt4, this set of elements being in parallel between the gate G and the anode A. The capacitive element Cd4 is a linking capacitive element that prevents the transmission of direct voltage from the anode to the gate. Its impedance is negligible, at the frequencies considered, when compared with that of the inductive element L3. The inductive element L3, the value of which is such that L3.Cga.w2=1, forms a high impedance anti-resonant circuit with the capacitive element Cga, w being the pulsation corresponding to the working frequency. This inductive element thus achieves the desired neutrodyning effect. The inductive element N3 is made in the form of a variable inductive element to enable the neutrodyning to be set for commissioning the amplifier to operate at a user-specified working frequency or to enable the neutrodyning operation to be started again at other frequencies if the user wishes to make his amplifer work at different frequencies.
The user of the amplifer has to make several settings when commissioning the equipment or when changing the working frequency, so as to achieve:
efficient neutrodyning, PA1 frequency tuning at the input tuning circuit L1, C1, PA1 frequency tuning at the output tuning circuit L2, C2,
These numerous settings entail several drawbacks. The first drawback, naturally, is the cost of the adjustable elements (variable inductive elements, variable capacitive elements, etc.) as compared with those of the fixed elements. The second and biggest drawback is the fact that each setting operation reacts on the other ones: a modification in the value of the inductive element L3 entails a modification in the tuning of the input circuit and vice-versa.
It is well known that the frequency setting of a neutrodyned amplifer is a delicate operation performed by successive approximations, and that only specialists can perform this operation swiftly. For example, when installing a radio-electrical transmission amplifier, it is the installer who performs this operation. If the setting goes wrong later any resetting by the user is often done very roughly and results in lower performance characteristics, either because of repeated hitches requiring action by the installer, or even because the components, and especially the grid tube, are destroyed.
An aim of the invention is to propose a high frequency amplifier assembly which does not have these drawbacks.